Why Add a Sub to Your PA?
There’s nothing quite like a chest-pounding kick drum, a bass line you can feel in your bones. But even with subtler applications, adding impact to low frequencies isn’t the only good reason to add a subwoofer to your PA system. Most subwoofers are designed to handle frequencies below 100Hz, taking the deepest bass frequencies out of the main PA speakers. While this doesn’t necessarily increase the volume of your PA, by spreading out the workload across multiple frequency ranges, you can add an amazing level of clarity to the sound of your PA. Ultimately, this increases the perceived loudness, without necessarily upping the SPL.
How to Integrate Your Subwoofer
There are two main ways to add a subwoofer to your PA: via a crossover network or via a special submix.
The Crossover Method
The more popular method (and the one used by virtually every house PA system) is to run the output of the mixing console through a crossover network, which splits the signal into multiple frequency bands and sends each band to a different amplifier/speaker. This method has the advantage of set-and-forget simplicity, in that once the system is configured, the mixing engineer doesn’t need to worry about the subwoofer. However, if the mix engineer isn’t diligent about using highpass filters on channels that don’t need extended bass, low-frequency noise projected from the subwoofer can quickly muddy up the mix.
The Aux Mix Method
The other method involves sending a dedicated aux (auxiliary) mix to the subwoofer, usually derived from an auxiliary send. In most cases, this submix will only include channels such as bass and kick drum, which contain significant low-frequency content. The advantage of this method is that only select sounds will receive the added bass boost, preventing low-frequency muddiness from creeping into the mix. However, due to the mix-specific nature of this method, it’s generally reserved for touring systems used repeatedly by the same engineers for the same band.
Active and Passive Subwoofers
Like other PA speakers, subwoofers come in both active/powered and passive/unpowered configurations. Both have their advantages and options. Here’s a basic rundown of each type.
Active/Powered Subwoofers
Like all powered PA speakers, active subwoofers include integrated power amplifiers, which make them easy to add to any PA rig, since you don’t have to worry about adding additional power amps to your rig. Powered subwoofers also frequently include active crossover circuits, allowing you to connect the stereo mix straight from the board and then send the highpassed signal on to the rest of the PA. The added amplifier and other electronics in an active sub add a bit of weight and cost over passives, but not having to add another power amp to your rig often more than makes up for the expense.
Passive/Unpowered Subwoofers
Most passive subwoofers are incredibly simple, featuring a large low-frequency driver and sometimes an induction circuit to roll off frequencies above their optimal operating range. This makes them comparatively lighter and less expensive, and you can pair passive subs with any sufficiently powerful amplifier. Chances are, you’ll need to invest in a crossover and an additional amplifier to add a passive subwoofer to your rig.
Crossovers
Crossovers come in a variety of complexity and formats. While there are internal passive crossovers inside some PA speakers, you’ll need an active crossover to split signal to your subwoofer and mains. Most powered subwoofers include active crossovers, but they tend to be simple and feature a fixed crossover point (usually around 100Hz). Outboard crossovers let you set the cutoff frequency and gain for each band. This makes outboard crossovers handy, even if you’re using active PA speakers. In addition to simple 2-way crossovers, more advanced 3- or 4-way crossovers can let you further split your signal to mid- and high-frequency speakers for even greater efficiency.
What to Look for in a Subwoofer
As diverse as PA subwoofers can be, there are a few features you’ll want to look out for.
Power/Power Handling
While power (active) and power handling (passive) don’t necessarily dictate the volume of your subwoofer, it’s often the best reference you’ll find for the volume of any given PA speaker. Typically, you’ll want your sub to match the same or slightly greater power output than your mains.
Speaker Size/Frequency Range
Just as power/power handling only suggests overall volume, the size of the speaker and listed frequency range provide a good idea of the depth and optimal operating range of a subwoofer. The bigger the speaker, the lower the frequencies you can expect from your sub.
Pole Mount
Many PA subwoofers include a pole socket on the top for mounting your mains. This is a particularly nice feature if you’re planning to stick a subwoofer at either end of the stage, both because it preserves floor space and because it helps you keep your speakers in alignment.
Casters
Subwoofers can be the heaviest gear in your live sound system. If you’re running a portable rig or frequently reconfiguring your setup, then a set of casters on your sub will save your back from unnecessary strain.
Connectors
In addition to considering the type of connectors on your sub (XLR, 1/4″, speakON, etc.), take a look at the type of I/O. Some subwoofers feature daisy chaining or stereo summing for use in 2.1 stereo sound reinforcement.
Crossover
If you’re getting a powered subwoofer, then it will probably come equipped with an active built-in crossover, but you’ll want to check to be sure. An adjustable crossover frequency can be a real bonus for tuning your rig. Some onboard crossovers also include a bypass switch, which lets you remove the circuit if you’re using an outboard crossover.
DSP
Onboard digital signal processing for integration with more sophisticated systems can also be extremely handy, particularly if you’re integrating your subwoofer with a more complex system. DSP options often include extra tuning options to better accommodate your main speakers, as well as remote control capabilities and more.